No.0027-Remove-Element

题目

Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. The relative order of the elements may be changed. Since it is impossible to change the length of the array in some languages, you must instead have the result be placed in the first part of the array nums. More formally, if there are k elements after removing the duplicates, then the first k elements of nums should hold the final result. It does not matter what you leave beyond the first k elements. Return k after placing the final result in the first k slots of nums. Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory.

解答思路

定义左右两个坐标，遍历数组，将左边存在val的元素，与右边不是val的元素相交换。 如果最后跳出循环的时候，l<=r。 如果这个时候l位置上的元素等于val，说明从l位置开始都是val，非val的数组长度就是l 如果这个时候l位置上的元素不等val，说明从l+1位置上开始都是val，非val的数组长度就是l+1

Python

class Solution:
  def removeElement(self, nums: List[int], val: int) -> int:
    if nums is None or len(nums) == 0:
      return 0
    l = 0
    r = len(nums) - 1
    while (l < r):
      while (l < r and nums[l] != val):
        l = l + 1
      while (l < r and nums[r] == val):
        r = r - 1
      temp = nums[l]
      nums[l] = nums[r]
      nums[r] = temp

    if (nums[l] == val):
      return l
    if (nums[l] != val):
      return l + 1
  # leetcode submit region end(Prohibit modification and deletion)